Objective：To investigate the expression of aquaporin 5 (AQP5) and MUC5AC in lung of bronchial asthmatic murine model and the regulatory effect of dexamethasone on the expression.Methods: Fortyeight female C57BL/6 mice were evenly randomized into control group,asthma group and dexamethasone group.The pathological changes of the lungs and the wet/dry weight ratios of the lungs were determined in mice of each group.The expression of AQP5 and MUC5AC mRNA in the lung was determined by realtime polymerase chain reaction (realtime PCR); the AQP5 protein was examined by Western blotting assay; the distribution of AQP5 protein in the pulmonary tissue was studied immunohistochemically; and the content of MUC5AC in the bronchial alveolar lavage fluid (BALF) was determined by ELISA.Results: Compared with the control group,the pulmonary expression of AQP5 in asthma group was obviously decreased (mRNA by\[42.5±3.6\]%,protein by \[64.3±8.2\]%),while the expression of MUC5AC was increased (mRNA by\[93.3±8.1\]%,protein by\[98.3%±7.2\]%) .Dexamethasone significantly decreased the pulmonary expression of AQP5 and MUC5AC compared with the asthma group (P<0.05).Conclusion: The decreased pulmonary expression of AQP5 in asthmatic mice might be related to the elevation of MUC5AC expression in the airway; dexamethasone can decrease AQP5 expression and therefore alleviate the overproduction of airway mucus,pulmonary inflammation and lung edema.